1. Field of Invention
This invention relates to a water-insoluble hydroxyalkylated cross-linked regenerated cellulose and a method of preparation thereof.
2. Description of the Prior Art
Regenerated cellulose is a well known product having a variety of uses. Most of the forms of regenerated cellulose, i.e. fibres, tubing, cord, particles, rods, films, membranes, etc. are relatively rigid and compact and of high density. Consequently their porosity is relatively low. Also, the product is incapable of holding very much water in its pores, i.e. the bulk of the product is mainly cellulose with a low pore volume so that its water regain value (g/g) and swollen volume (ml/g) are disadvantageously low. Water Regain Value (Wr) is defined as EQU Wr=Ww/Wg
where Ww is the weight of water absorbed by a weight Wg of dry gel. The absorbed water is that imbided within a gel particle but does not include the water between the particles.
Many applications of polysaccharide materials require them to have useful water regain values as it is this water which is held in the pores which gives the product its utility. Two such uses are the desalting of protein solutions with wet polysaccharide particles and the concentration of protein solutions using dry polysaccharide particles. Both rely on the small molecules, salts and water, respectively entering the pores of the polysaccharide but not the larger protein molecules.
Still other applications require the polysaccharide materials to have reasonable porosity to high molecular weight compounds. One such application is in gel chromatography. This is a process of separation based on the ability of different substances to pass through a gel bed at different speeds according to their molecular size. The gel bed is commonly a column of polysaccharide particles. The lower molecular weight substances are able to take greater advantage of the water in the pores of the particles and so are eluted from the column after the higher molecular weight substances. In this way proteins of different molecular weight can be separated. Conventional methods of regenerating cellulose have not allowed the use of regenerated cellulose in these areas because of the disadvantages set out above.
Recently these disadvantages have been overcome by the regeneration of cellulose into more swollen forms of low cellulose content and with greatly increased porosity. This has been achieved by regenerating the cellulose solution in an organic solvent immiscible with the solvent for the cellulose (water), rather than the conventional solution of regenerating agent in water. In this way the cellulose is prevented from undergoing a strong shrinkage in the regenerating process, thereby causing the cellulose to become compact and attain a high density. The product prepared by such a method has been shown to be useful as a separating medium for gel chromatogrphy (U.K. Pat. No. 1,234,099) and as a matrix for the attachment of ion exchange groups to produce ion exchangers with high capacity for the adsorption of a protein (U.K. Pat. No. 1,293,611). These ion exchangers show considerable improvement in protein capacity over similar ion exchange derivatives of particles of cellulose regenerated by conventional means (U.S. Pat. No. 3,573,277), especially towards proteins of high molecular weight, i.e. greater than 50,000-100,000. This results from their high swollen volume and high porosity.
It has now been shown to be possible to prepare low density forms of regenerated cellulose by derivatising the cellulose with hydroxyalkyl and cross-linking groups. The products so obtained have both a higher swollen volume and increased chemical reactivity as compared with cellulose regenerated by conventional means.
Hydroxyethyl cellulose gels have been prepared from hydroxyethyl cellulose by forming an emulsion of an aqueous hydroxyethyl cellulose solution in an organic solvent and then cross-linking the hydroxyethyl cellulose in the microdrops formed to produce water insoluble beads (W. Brown and K. Chitumbo, Chemica Scripta 1972, 2, 88-92). This process is analogous to that used for dextran to produce beads of cross-linked dextran.
Hydroxypropyl cellulose (an insoluble product) has been prepared from ordinary cellulose (fibrous cellulose powder) by reaction with propylene oxide and epichlorohydrin in the presence of sodium hydroxide (B. Alme and E. Nystrom, J. Chrom. 1971, 59, 45-52). The method used was to treat sodium cellulose with a large excess of propylene oxide and epichlorohydrin under reflux.
In neither case has there been used a regenerated cellulose giving rise to a product with the same properties as the present product of high swelling while remaining water insoluble.